Conventional methods for molding hollow containers made of synthetic resins include, for example, a technology which obtains a preform by injection molding or extrusion, and blow-molds the preform by a blow mold unit provided in a blow molding device to form a hollow container. A known example is a technology in which a preform is disposed within a molding space (cavity) formed by a blow mold provided in a blow mold unit, and the preform is blow-molded within this cavity for expansion to form a hollow container (see, for example, Patent Document 1).